Devices for roundness measurement generally measure roundness by mounting a cylindrical object or any object with a circular cross-section onto a rotatable mount and contacting the surface of the object with measuring probes. The displacement of the measuring probes during rotation of the object is measured to obtain the contour of the circular cross-section of the object.
Conventional roundness measurement devices can be divided into two categories: a rotary sensor type with a fixed object and a rotary table type with fixed probes. A spindle is a required element for both types of device; consequently, spindle errors are present inevitably even with high-precision roundness measurement devices.
Presently, the most common method for roundness measurement is to clamp an object onto the center of a rotary table and assess the radial variation in object contour during rotation by measuring a single contact or noncontact displacement. However, errors caused by radial run-out of the rotating spindle and center positioning, of the object to the spindle are usually ignored.
More specifically, as current roundness measurement devices adopt only one probe, measurement of roundness is conducted under the assumption that no spindle run-out and object's centering error would occur when the object rotates for 360 degrees. Therefore, when an object is mounted onto a rotating table and the object is not aligned with spindle of the rotating table, only radius variation, but not the actual diameter and radius, of the object can be obtained.